In recent years, in order to realize semiconductors and liquid crystal displays having a high-performance and high-throughput, a plasma process has become indispensable for manufacturing these products. Although there are various methods for plasma excitation, a parallel plate type RF plasma excitation apparatus or a inductive coupling type plasma apparatus has been used to manufacture semiconductors or liquid crystal displays. These plasma apparatuses have several essential problems in that a large damage is given to a device and a high-performance process at a high-speed cannot be achieved. Accordingly, it has become difficult to satisfy demands of semiconductors and liquid crystal displays to have a high-performance and high-throughput
Accordingly, a microwave plasma apparatus has recently been attracting attention, which can excite high-density plasma by a microwave electric field without using a direct current magnetic field. As such kind of micro plasma apparatus, an apparatus (Japanese Laid-Open Patent Application No. 9-63793) is known, which excites plasma by ionizing a gas in a vacuum chamber by a microwave electric field generated by a microwave emitted to the vacuum chamber from a flat antenna (slot antenna) having many slots that are arranged to generate a uniform microwave. Additionally, there is also known an apparatus (WO98/33362), which excites plasma by introducing a microwave, which is emitted by a slot antenna provided outside a vacuum chamber, into the vacuum chamber by being passed through a dielectric material separation wall and a dielectric material shower plate. Since the microwave plasma excited by those methods has a high-density and a low electron temperature, a process having no damage at a high speed can be performed. Additionally, since uniform plasma can be excited even on a large area substrate, it can be easily dealt with an increase in the size of a semiconductor substrate or a liquid crystal display.
However, these conventional microwave plasma apparatuses have a problem in that a substance, which is produced by dissociation and combination of a process gas due to the plasma, adheres onto a surface of the dielectric material separation wall or the shower plate. If a film having a low resistivity is deposited on the surface, the microwave is reflected, and if a film having a high resistivity, the microwave is absorbed. Accordingly, the plasma excitation power is decreased due to adhesion of the substance onto the surface of the dielectric material separation wall or the dielectric material shower plate, which reduces the plasma density and deteriorates stability of the plasma. In the worst case, it becomes a situation in which the plasma cannot be exited. In order to eliminate such a problem, it is necessary to frequently perform a chamber cleaning and maintenance so as to remove the adhered film, which significantly decreases the throughput.
In the reactive ion etching which is indispensable for producing semiconductors or liquid crystal displays, anisotropic etching is achieved by irradiating ions in the plasma onto a substrate surface by accelerating up to 100 eV by an electric filed in a sheath formed between the substrate and the plasma. In order to generate a direct current voltage (self bias voltage) for accelerating the ions to a desired energy in the sheath near the substrate, an RF wave ranging from about several hundreds KHz to about several tens MHz is applied to the substrate. Since the plasma can be regarded as a conductive material, the RF voltage applied to the substrate is divided into that the sheath near the substrate and the sheath near the grounded part. That is, if the RF wave is applied to the substrate, the RF voltage is applied not only to the sheath near the substrate but also to the sheath near the grounded part, and, thereby, the DC voltage of the sheath near the grounded part is increased and a plasma potential is increased. If the plasma potential becomes greater than 15 to 30 V, contamination occurs due to sputtering of the surface of the grounded part due to bombardment of the accelerated ions.
A ratio of the RF voltages applied to the sheath near the substrate and the sheath near the grounded part is determined by impedances of these sheathes. If the impedance of the sheath near the grounded part is much smaller than the impedance of the sheath near the substrate, a most part of the RF voltage applied to the substrate is applied to the sheath near the substrate. That is, if the area of the grounded part to which the plasma contacts is sufficiently larger than the area of the substrate electrode (normally, more than four times), the plasma potential is not increased when a RF wave is applied to the substrate. Thus, a problem associated with the contamination due to the sputtering can be avoided. Additionally, a large DC voltage can be efficiently generated in the sheath near the substrate.
However, in the conventional microwave plasma apparatus, since the opposing surface of the substrate is covered by a dielectric material in its entirety, the area of the grounded part to which the plasma contacts cannot be large. Normally, an area of the grounded part to which the plasma contacts is less than about three times the area of the substrate electrode. Accordingly, it is difficult to apply to an reactive ion etching such as a process in which a high energy ions must be bombarded to a substrate surface.
In a process for forming a thin film containing a metal such as metal thin film, feroelectric film, and high dielectric thin film by CVD (chemical vapor deposition) method, and an organometallic gas which is a compound of metal atoms and organic molecules is used. If the bonds between the metal atoms and the organic molecules is selectively cut off, a thin film having a good characteristic which causes no impurity contamination will be formed. However, if the organic molecules are decomposed, a large amount of carbon impurity atoms are mixed into the film, which deteriorates the characteristic of the thin film. Additionally, in the etching process, if the dissociation of the process gas progresses in excess, the selectivity between the film to be etched and a resist mask or the underlying material is deteriorated, and it becomes difficult to etch a fine pattern having a large aspect ratio. In the conventional microwave plasma processing apparatus, the process gas is directly introduced into an area close to a position at which the microwave is incident and having a high plasma density and a relatively high electron temperature. Thereby, the dissociation of the process gas progresses in excess, and a good result cannot be obtained in formation of a thin film using an organometallic gas or fine pattern etching.
When a microwave is incident on plasma, the microwave propagates in the plasma if the electron density if smaller than the cutoff density nc represented by the following equation.nc=∈0ω2m0/e2 
where ∈0 is a permittivity of dielectric ratio of vacuum; ω is microwave angular frequency, m0 is a mass of an electron, and e is a charge of an electron. On the other hand, if the electron density is higher than the cutoff density, the microwave is reflected in the vicinity of a plasma surface. At this time, the microwave penetrates into the plasma by a penetration length (normally, several millimeters to ten millimeters), and gives energy to electrons in the plasma so that the plasma is maintained. In to the microwave plasma excitation, if the electron density is lower than the cutoff density, uniform and stable plasma cannot be excited due to dispersion of the microwave in the chamber. In order to excite uniform and stable plasma, it is indispensable to reflect a large part of the microwave by exciting plasma having an electron density sufficiently higher than the cutoff density in the vicinity of the surface on which the microwave is incident. In order to excite a stable plasma having a high electron density, an inert gas such as Ar is preferably used as the plasma excitation gas. If a gas other than a monatomic molecule gas is added, it tends to deteriorate the stability of the plasma due to the electron density being decreased since the energy of the microwave is used for dissociation of the gas molecules. In the conventional microwave plasma apparatus, since only a small amount (several percent) of gas other than the inert gas can be added, there is a problem in that process window is narrow and it cannot deal with a high speed process.
When the electron density in the vicinity of the plasma surface is higher than the cutoff density, a large part of the microwave incident on the plasma is reflected in the vicinity of the surface. The reflected wave is received by the slot antenna, and, thereafter, emitted from the slot antenna by being reflected by a matching unit connected between the slot antenna and the microwave power source. The microwave gradually provides its energy to the plasma while repeatedly reflected between the plasma surface and the matching unit. That is, the microwave is in a resonant state in a part between the plasma surface and the matching unit. Accordingly, a high energy density microwave is present in this part, and a large loss is caused due to a small conductive loss of a metal wall of the waveguide or a small dielectric loss of the dielectric material. In the conventional microwave plasma apparatus, these losses are large, and, thereby, the plasma excitation power efficiency was low. Additionally, if a large power microwave is supplied so as to obtain a high-density plasma, an arc discharge is generated in a slot part formed on the surface of the slot antenna. Thereby, there is a problem in that the antenna is broken or a discharge occurs in a gas passage between the dielectric material separation wall and the dielectric material shower plate.